Elizabeth was a widow when she married Thomas, although that is not recorded in the marriage register. Her first marriage was to Edward Grimmit from Aveley in Essex; a clandestine marriage at the Fleet on 13 July 1729. Elizabeth and Edward had two children baptised at Southfleet; Thomas bap 30 July 1732 and Sarah bap 19 Nov 1733. Edward was buried there on 13 July 1736.
Elizabeth then married Thomas Holdaway on 14 October 1741 at Southfleet. A Settlement certificate dated 08 March 1741/42 from Southfleet gives his parish of settlement as Ash.These dates fit very well; a young single man was no financial threat to any parish, but a married man with a child on the way was.
There is no baptism record for Thomas in Ash, so it's likely that his settlement there was gained through employment.There are no other (known) Holdaways in Kent prior to Thomas so it seems likely that he migrated from further afield, the name is found mainly in Hampshire with a few instances in London.
Elizabeth's origins are also obscure. Horton Kirby parish register has the baptism of an Elizabeth Millis on 16 Aug 1696 daughter of G Millis, whoever he was, but she would have been too old to have had a baby in 1750.
Children of Edward Grimmit and Elizabeth:
Thomas bap 30 July 1732, Souhfleet
Sarah bap 19 Nov 1733, Southfleet
Children of Thomas Holdaway and Elizabeth:
Edward bapt 12 September 1742, Southfleet
William bapt June 1746, Ash
Elizabeth bapt 25 November 1750, Southfleet, bur 09 January 1790, Stone
The Ash parish register shows their son William's parents as Thomas and Sarah. All things considered I think that that is probably an error in the register; there are no other Holdaways in the area at that time, Thomas and Elizabeth spent most of their time in Southfleet even though his parish of legal settlement was Ash. For whatever reason William was Christened at Ash where the family were maybe not so well known to the vicar/clerk who simply recorded the wrong name for the child's mother - maybe he confused her name with that of her daughter.
Elizabeth was buried 04 May 1780 at Southfleet, Thomas's burial has not yet been found.